Without limiting the scope of the invention, its background is described in connection with composite materials.
Graphene is one of the strongest materials ever tested. Measurements have shown that graphene has a breaking strength 200 times greater than steel, with a tensile modulus (stiffness) of 1 TPa (150,000,000 psi). An atomic Force Microscope (AFM) has been used to measure the mechanical properties of a suspended graphene sheet. Graphene sheets, held together by van der Waals forces, were suspended over SiO2 cavities where an AFM tip was probed to test its mechanical properties. Its spring constant was in the range 1-5 N/m and the Young's modulus was 0.5 TPa (500 GPa) thereby demonstrating that graphene can be mechanically very strong and rigid. Despite these nanoscale mechanical properties, Graphene has not been able to transition to a macro-scale mechanical structure. Various research institutes have loaded plastic/polymer/epoxy with carbon nanotubes (CNT), graphene flakes (GF), and graphene oxide (GO) and seen up a 200% increase in tensile strength in the loaded plastic/polymer/epoxy. The process of producing a loaded plastic/polymer/epoxy does not translate to a viable composite structure. The inability to translate the technology to a viable composite structure is combination of technical issues and cost factors. The technical limitation includes stochastic process in the curing of the plastic/polymer/epoxy that results in random shrinkage phenomena that is exacerbated in larger composite structures/devices. The distribution of the laded mechanical structural materials (CNT, GF, and GO) is non-uniform creating weak regions and failure points in the loaded plastic/polymer/epoxy material. The superior properties of graphene compared to polymers are also reflected in polymer/graphene nanocomposites. Polymer/graphene nanocomposites show superior mechanical, thermal, gas barrier, electrical and flame retardant properties compared to the neat polymer. Improvement in the physicochemical properties of the nanocomposites depends on the distribution of graphene layers in the polymer matrix as well as interfacial bonding between the graphene layers and polymer matrix. The combination low yield and high cost of the CNT, GF, and GO materials makes the approach not viable. Interfacial bonding between graphene and the host polymer dictates the final properties of the graphene reinforced polymer nanocomposite.
Graphene is an allotrope of carbon. Graphene's structure is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb or hexagonal crystal lattice. The carbon-carbon bond length in graphene is about 1.42 Å. Graphene sheets stack to form graphite with an inter-planar spacing of 3.35 Å. Multiple graphene sheets/flakes are bonded together by van der Waals forces.
Graphene can be oxidized by a number of processes including thermal, chemical or chemical-mechanical. Reduction of graphite oxide monolayer films e.g. by hydrazine, annealing in argon/hydrogen was reported to yield graphene films of low quality.
Graphene can be produced in significant quantities from microcrystalline graphite that is treated with a mixture of acids such as sulfuric, nitric, and other oxidizing chemical in combination mechanical and/or thermal energy elements. This processing will produce graphene flakes ranging from a few nanometers to tens of microns depending and the specific processing environment. If one uses a shaker mill in conjunction with an oxidizing agent the time duration in the mill will determine the size of the flake of graphene. In general, the longer the processing time in the mill the smaller the graphene flake. The oxidizing process produces a carboxyl group on the perimeter of the graphene flake. The resulting graphene flakes can be on the order of 5 Å in thick and can be suspended in a number of solutions including but not limited to: tetrahydrofuran, tetrachloromethane, water, and/or dichloroethane.